This invention relates to thermoplastic bottles and more particularly to thermoplastic beer and carbonated beverage bottles having a unique combination of physical properties.
There are polymeric materials available today which have high impact strength and low gas permeability, but none of these possesses the property of excellent resistance to creep strain under tensile load, which is so necessary to the manufacture of commercially acceptable plastic bottles for beer and carbonated beverage. Creep strain is undesirable for two reasons: it results in a change of bottle shape whereby the liquid level is lowered, and it results in a loss of carbonation of the liquid owing to the expanded volume of the bottle.
The instant invention therefore provides a thermoplastic bottle having the following unique combination of physical properties: high impact strength, excellent resistance to creep strain under tensile load, and extremely low gas permeability. Existing thermoplastic bottles possess one or two of these properties, but the combination of all three in the required degree is unknown in the prior art.
This unique combination of physical properties is desired for the packaging of fluids under a high internal pressure, i.e., beer, carbonated beverages, and aerosol container products. A high level of molecular orientation, as characterized by orientation release stress (ASTM D 1504), is utilized to obtain the physical properties specified above.
It is known that the physical properties of thermoplastic polymers can be improved by uncoiling and straightening the molecules of the polymeric material by a stretching operation while the polymeric material is at a temperature at which such molecular orientation can be imparted thereto (orientation temperature), i.e., while the polymeric material is in the so-called "leathery" state, and thereafter cooling the material so that the molecules of the polymer are set in the direction or directions in which the stretch is applied.